If other factors are equal, four-speed transmissions typically offer worse fuel efficiency than transmissions with more gears. However, they offer better efficiency than three-speed transmissions, which are rare on newer cars.
Car engines can operate at a wide range of rotation speeds, but they are designed to operate most efficiently within a relative small range of rotation speeds. When driven above this range, engines use significantly more fuel. When driven below it, acceleration is limited and fuel efficiency can suffer. As a result, cars rely on multiple gearings to keep them within this band as much as possible. Four-speed transmissions have long been viewed as sufficient for attaining adequate fuel efficiency.
While offering more gears can provide better efficiency, automatic transmissions take time to switch between different gears, and vehicles can't provide power to the wheels while in transition. Part of the appeal of four-speed transmissions is that the vehicle doesn't spend much time switching between gears, although faster transmissions have partially alleviated this disadvantage.
Not all vehicles need discrete gearings. Continuously variable transmissions use a belt system to adjust the gearing seamlessly as the car speeds up, allowing it to operate efficiently without have to switch gears. Electric motors provide the same efficiency regardless of how fast they spin, so adjustable gearing doesn't lead to improved efficiency.